Numerous forms of photographic, self-developing assemblages have been proposed in the prior art as well as being sold commercially, including both roll film and individual film units. The type of film assemblage or unit with which the present invention is concerned generally comprises a pair of sheets, one of which serves as a support for a photosensitive image-recording medium and a rupturable container of a viscous liquid processing agent adapted to be distruibted in a thin layer in contact with an exposed area of the photosensitive medium by superposing the sheets and avdancing them between a pair of pressure-applying members to distribute the processing liquid between the sheets for permeation into the recording medium. The photosensitive sheet (carrying the photosensitive medium) and the other or second sheet may be separated or superposed during exposure, are superposed during processing, and may be separated or allowed to remain laminated to one another by the processing liquid subsequent to processing; or they may be retained in superposed relation prior to, during, and subsequent to exposure and processing. Film units of this latter type are disclosed, for example, in U.S. Pat. Nos. 3,415,644, 3,415,645, 3,415,646, 3,473,925, 3,550,515, 3,578,540, 3,589,904, 3,594,164, 3,594,165, 3,607,285, 3,615,421, 3,615,436, 3,615,539, 3,615,540, 3,619,192, 3,619,193, 3,621,768, 3,652,281, 3,652,282, 3,672,890, and 3,689,262.
Whatever the basic format and structure of the film assemblage, it is important, particularly in film units of the type described in the aforementioned U.S. patents, to spread the processing liquid in a layer of uniformly predetermined thickness throughout the area exposed and processed, preferably to produce an image visible through one of the sheets. One approach to liquid spreading and spread thickness control is to employ juxtaposed pressure-applying members, particularly rollers, resiliently biased toward one another and move the film unit therebetween to first eject the liquid from a rupturable container attached to the sheets near the leading end of the area to be processed and then progressively spread the mass of ejected liquid toward the trailing end of this area and the sheets. Liquid spread thickness control is achieved by providing sheet-like spacing elements at least at the lateral margins of the sheets for separating the medial portions of the pressure-applying members and sheets in the area to be processed in order to provide space between the sheets in which the liquid is spread. Heretofore, such spacing means have taken the form of uniformly thick layers or sheets formed of such materials as paper, organic plastics, and metallic foil, or combinations thereof, in the form of sheets or layers having openings defining the image area or strips and/or combinations of strips and sheets adapted to define the area to be processed.
In the film unit structure described in the aforementioned patents, these spacing means are designed to perform a number of functions in addition to spread thickness control including masking of non-image areas e.g. forming a border, and securing the two sheets to one another at at least their lateral and trailing end margins. For this purpose, the spacing means take the form of a generally rectangular sheet formed with a rectangular exposure and/or viewing opening surrounded on at least three sides by lateral and trailing end marginal portions secured to one of the photosensitive and second sheets, folded around the lateral and trailing end edges of the sheets and secured to the other of the photosensitive and second sheets.
The copending application of Edwin H. Land, Ser. No. 250,611 now U.S. Pat. No. 3,761,268, relates to such film units wherein the spacing means is tapered to permit the developing liquid to be applied as a layer of more uniform thickness to obviate the problems which are inherent when the developing liquid is not so applied in a uniformly predetermined thickness. The spacing means of this copending application may be in the form of a binder sheet of known function having a central opening through which the film unit is photoexposed and/or through which the resulting image is to be viewed, the sheet being tapered from a maximum thickness near the leading edge (edge which exits from the exposure apparatus first) of the central opening to a minimum thickness at a point between the trailing edge of the central opening and the trailing edge of the sheet. This taper from maximum to minimum thickness may be on the order of 0.30 mil .+-. 0.03 mil.